knock on wood

To knock on something made of wood to keep from having bad luck. — Many people believe that you will have bad luck if you talk about good luck or brag about something, unless you knock on wood; often used in a joking way.

[knock it off] {v. phr.}, {slang}, {informal} 1. To stop talking about something considered not appropriate or nonsensical by the listener. - Used frequently as an imperative. * /Come on, […]A Dictionary of American Idioms